refactor: reorganize template write paths + add frontmatter + manifest

All skill templates now write artifacts to subdirectories under
$PROJECTS_DIR/$SLUG/ (reviews/, plans/, reports/). Adds YAML
frontmatter to markdown artifacts and manifest-append calls.

Templates updated:
- plan-eng-review: plans/ + reviews/
- plan-ceo-review: plans/ceo/ + reviews/
- plan-design-review: reviews/
- ship: reviews/
- design-review: reports/
- qa: reports/
- qa-only: reports/
- review/greptile-triage: uses gstack-slug instead of remote-slug

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ship/SKILL.md.tmpl

@@ -61,7 +61,7 @@ If the Eng Review is NOT "CLEAR":
 1. **Check for a prior override on this branch:**
    ```bash
    eval $(~/.claude/skills/gstack/bin/gstack-slug 2>/dev/null)
-   grep '"skill":"ship-review-override"' ~/.gstack/projects/$SLUG/$BRANCH-reviews.jsonl 2>/dev/null || echo "NO_OVERRIDE"
+   grep '"skill":"ship-review-override"' $PROJECTS_DIR/$SLUG/reviews/$BRANCH.jsonl 2>/dev/null || echo "NO_OVERRIDE"
    ```
    If an override exists, display the dashboard and note "Review gate previously accepted — continuing." Do NOT ask again.
 
@@ -75,7 +75,7 @@ If the Eng Review is NOT "CLEAR":
 3. **If the user chooses A or C,** persist the decision so future `/ship` runs on this branch skip the gate:
    ```bash
    eval $(~/.claude/skills/gstack/bin/gstack-slug 2>/dev/null)
-   echo '{"skill":"ship-review-override","timestamp":"'"$(date -u +%Y-%m-%dT%H:%M:%SZ)"'","decision":"USER_CHOICE"}' >> ~/.gstack/projects/$SLUG/$BRANCH-reviews.jsonl
+   echo '{"skill":"ship-review-override","timestamp":"'"$(date -u +%Y-%m-%dT%H:%M:%SZ)"'","decision":"USER_CHOICE"}' >> $PROJECTS_DIR/$SLUG/reviews/$BRANCH.jsonl
    ```
    Substitute USER_CHOICE with "ship_anyway" or "not_relevant".